1. Field of the Invention
The present invention relates to an image forming method and apparatus in which an image carrier belt is driven and moved only by a friction force from a driving roller and the image carrier belt can be driven and moved stably.
2. Discussion of the Background
Image forming apparatuses such as a copying machine, a printer, a facsimile machine or a multi-function machine having at least two of a copying machine, printer and facsimile machine functions, have been known. The form of an endless image carrier belt used in the image forming apparatuses can be set somewhat freely as compared to an image carrier drum, and therefore devices such as a charging member, etc., arranged around the image carrier belt can be arranged relatively freely. Further, because the charging member contacts the image carrier belt to charge the image carrier belt, the generated amount of ozone can be decreased as compared to a corona discharging device having a charging wire which is arranged far apart from the image carrier belt. As a concrete structure of an image forming apparatus using the image carrier belt, there is one including an endless image carrier belt which is spanned around a driving roller and a driven roller, and which is moved and driven by a friction force from an outer circumference surface of the rotating driving roller, and a charging member to charge the image carrier belt by contacting the outer surface of the image carrier belt, and in which an electrostatic latent image is formed on the image carrier belt charged by the charging member, and the electrostatic latent image is visualized as a toner image, and further the image toner is transferred directly or via an intermediate transferring member onto a recording medium. As the charging member, for example, a charging roller, a charging blade or a charging brush, etc., which contacts an outer surface of the image carrier belt at least at a part thereof, can be adopted.
Because all of the charging members described above contact the outer surface of the image carrier belt, when the image carrier belt moves the friction force between the image carrier belt and the charging member provides a braking force on the image carrier belt. On the other hand, the image carrier belt is driven and moved by the friction force from the outer circumference surface of the driving roller, and therefore when the braking force described above is provided on the image carrier belt, the image carrier belt easily slips with respect to the driving roller. If the slip amount is large, the moving stability of the image carrier belt is reduced, and the linear speed of the belt surface changes, causing unevenness in the belt moving speed. If the unevenness occurs in the image carrier belt moving speed, density unevenness occurs in a toner image transferred onto a recording medium. Further, in an image forming apparatus in which toner images of different colors are formed on the image carrier belt in sequence, transferred onto an intermediate transferring member one after another while superimposing one upon the other, and the superimposed toner images are transferred onto a recording medium all together, the unevenness in the belt moving speed described above causes color dislocation in the color toner image transferred onto the recording medium, thereby reducing the image quality.
If holes are formed in a side edge portion of the image carrier belt over its entire length at equal intervals, and pins formed in a driving roller over its entire circumference at equal intervals and are engaged with the holes, the image carrier belt can be forcefully driven by the pins when the image carrier belt is driven and moved by the rotation of the driving roller. Therefore, in this case, even when the charging member contacts the outer surface of the image carrier belt, slip of the image carrier belt with respect to the driving roller can be effectively reduced, and thereby density unevenness or color dislocation of a toner image can be prevented from occurring. However, if a driving roller having many pins and an image carrier belt likewise having many holes are used, the structure of an image forming apparatus is complicated and the cost is high.
Accordingly, an object of the present invention is to provide an image forming apparatus in which an image carrier belt is by driven and moved only by a friction force from a driving roller and the image carrier belt can be driven and moved stably.
Those and other objects and advantages are achieved by the present invention which provides an image forming apparatus including an endless image carrier belt configured to be spanned around a driving roller and a driven roller and moved by a friction force provided from an outer circumference surface of the driving roller when the driving roller is rotated, a charging device configured to contact an outer surface of the image carrier belt to charge the image carrier belt; and wherein the outer surface of the image carrier belt which the charging device contacts is a portion of the image carrier belt contacting the driving roller.